In the design of solenoids having latching coils there is a specified direct current (VDC) required to both latch ON and latch OFF the coil, thus either opening or closing the solenoid valve. We will assume that the coil is 12 VDC. Therefore, a 12 VDC current would be required to latch ON or open the solenoid valve. However, since the coil is of a latching nature, there is no need for continuous current to energize the coil. The basic principle of the latching coil is that a magnetic draw is present at both the open and closed positions of the coil. The VDC current applied to the coil increases the magnetic draw at the open end thus drawing the piston of the solenoid body towards the upper end of the coil or open position. By inverting this VDC current the magnetic draw is reversed, thus moving the piston of the relay to the lower or closed position. In both the on and off positions, the coil will maintain the piston's position without the need of continuous current.
A Programmable Logic Controller (PLC) or similar device cannot typically be programmed to transmit a brief voltage signal to the solenoid coil in order to latch on the solenoid valve and then transmit an inverted signal to latch off the valve. A continuous signal creates excess heat at the coil and unneeded energy expense.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale. For clarity non-essential reference numbers are left out of individual figures where possible.